Earnings (Loss) per Share - Computation of Basic and Diluted Earnings per Common Share (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Basic EPS:
Net income	$ 258	$ 624	$ 803	$ 234	$ 289	$ 737	$ 950	$ 700	$ 1,919	$ 2,676	$ 7,610
Weighted average common shares outstanding	477,165	484,772	490,818	503,902	514,571	525,415	563,000	606,245	489,164	552,308	668,393
Basic EPS (in dollars per share)	$ 0.54	$ 1.29	$ 1.64	$ 0.46	$ 0.56	$ 1.40	$ 1.69	$ 1.15	$ 3.92	$ 4.85	$ 11.39
Diluted EPS:
Net income for purposes of computing diluted EPS									$ 1,919	$ 2,676	$ 7,610
Share computation for diluted EPS (in thousands):
Basic weighted average common shares outstanding	477,165	484,772	490,818	503,902	514,571	525,415	563,000	606,245	489,164	552,308	668,393
Dilutive effect of stock awards									2,528	3,791	18,962
Diluted weighted average common shares outstanding	479,382	486,625	492,965	507,797	518,358	528,510	566,040	611,488	491,692	556,099	687,355
Diluted EPS (in dollars per share)	$ 0.54	$ 1.28	$ 1.63	$ 0.46	$ 0.56	$ 1.40	$ 1.68	$ 1.14	$ 3.90	$ 4.81	$ 11.07
Restricted stock unit awards excluded from the calculation of diluted EPS because inclusion would be antidilutive									328	1,429	764